According to NOAA:

Each day in the United States over 210 million weather observations are processed and used to create weather forecasts. These measurements are obtained from automated instruments, professional meteorologists, and thousands of trained volunteer observers. Observations are recorded and uploaded into powerful computer models that create global and regional weather forecasts. Meteorologists in the 126 National Weather Service local offices combine these large scale forecasts with local observations and their knowledge of local weather patterns to make a forecast for their specific region. 

That being understood, the forecast is based on a wide area.  Many times, we have experienced great weather while just few miles away may be getting heavy rain.